Dehydrogenation of alcohols under ambient atmosphere by a recyclable sol-gel encaged iridium pincer catalyst

Article abstract of DOI:10.1016/j.catcom.2012.01.017

We demonstrate that a sol-gel entrapped iridium-pincer catalyst 1 is capable of promoting acceptorless dehydrogenation of primary and secondary alcohols. Although the dehydrogenations by the non-heterogenized catalyst are faster than by the sol-gel encaged complex, recyclability and significant stabilization of the organometallic species toward non-inert conditions represent a clear advantage from the environmental and practical points of view.

Full text of DOI:10.1016/j.catcom.2012.01.017

Catalysis Communications 20 (2012) 68–70
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Dehydrogenation of alcohols under ambient atmosphere by a recyclable sol–gel
encaged iridium pincer catalyst
⁎
⁎⁎
Kobi Oded, Sanaa Musa, Dmitri Gelman , Jochanan Blum
Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
We demonstrate that a sol–gel entrapped iridium-pincer catalyst 1 is capable of promoting acceptorless dehy-
drogenation of primary and secondary alcohols. Although the dehydrogenations by the non-heterogenized
catalyst are faster than by the sol–gel encaged complex, recyclability and significant stabilization of the
organometallic species toward non-inert conditions represent a clear advantage from the environmental
and practical points of view.
Received 10 November 2011
Received in revised form 3 January 2012
Accepted 16 January 2012
Available online 24 January 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Sol–gel processes
sp3-Metalated pincer complex
Acceptorless dehydrogenation
Iridium
1. Introduction
TON of 3.6*103, but it has two major disadvantages: it is sensitive to
air, and it cannot be reused.
Many important commercial carbonyl compounds are fabricated
by catalytic dehydrogenation of primary and secondary alcohols
[1–4]. From an environmental point of view, dehydrogenation
reactions that take place in the absence of hydrogen acceptors are
of particular interest [5].
In their seminal reports, Robinson [6] and Cole-Hamilton [7]
suggested the application of ruthenium-based catalysts for high-
temperature dehydrogenation of alcohols to ketones and dihydrogen.
More recently, important progress in mild acceptorless dehydrogena-
tion of alcohols has been achieved [8–15]. For example, a series of
new catalysts relying on transition metal complexes coordinated to
sophisticated non-innocent ligands was described to exhibit excellent
activity under milder reaction conditions [8,16–24]. Furthermore, it
was demonstrated that simple combination of commercially available
metal precursors and structurally more simple ligands can be as effi-
cient as well [25].
In this study we report on the heterogenization of this iridium
complex within a silica sol–gel matrix. The choice of silica sol–gel
for the immobilization of the catalyst was based on the ability of the
ceramic material to host a great variety of chemicals and to assist dif-
ferent catalytic reactions (see e.g., Ref. [27]). The catalyst entrapment
is usually performed either by covalent binding to the backbone of the
silica support or by “physical” encagement, in which the catalyst is
bound to the sol–gel host by hydrogen bonds, Van der Waals, or other
relatively weak interactions. The latter method of entrapment not
only is often efficient for avoiding metal leaching during the catalytic
processes, but also stabilizes the catalyst against unwanted oxidation
and hydrolytic reactions. The entrapment of the catalyst within silica
sol–gel matrices enables usually its recovery and facile recycling in
further runs without significant loss of catalytic activity.[27–30]
2. Experimental section
Recently, we reported on an acceptorless system based on a transi-
tion metal derived iridium-based PC(sp3)P-metalated pincer catalyst
in which both the metal and the skeleton of the complex play an active
role in the catalytic reaction (Scheme 1) [26].
The iridium-containing catalyst, 1, proved efficient for the transfor-
mation of a variety of primary and secondary alcohols demonstrating
2.1. Preparation of the heterogenized catalyst
A mixture of tetramethoxysilane (TMOS 3.4 mL), triply distilled
water (TDW 2 mL), and MeOH (2.4 mL) was stirred at room temper-
ature (24 °C) for 15 min. Separately a mixture of complex 1 (30 mg,
0.0347 mmol), THF (4 mL, 50 mmol), benzene (1 mL) was stirred at
24 °C for 10 min. The two mixtures were united and stirred for 24 h.
The resulting gel was aged for another 24 h and dried on P2O5 at
0.5 Torr to achieve constant weight. The resulting yellow material was
washed and sonicated with THF (2×20 mL) for 30 min. The washings
were subjected to Inductively Coupled Plasma (ICP) analysis. The Ir
⁎
Corresponding author. Tel.: +972 2 6584588; fax: +972 2 6585279.
⁎⁎ Corresponding author. Tel.: +972 2 6585329; fax: +972 2 6513832.
E-mail addresses: dgelman@chem.ch.huji.ac.il (D. Gelman),
jblum@chem.ch.huji.ac.il (J. Blum).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.017

K. Oded et al. / Catalysis Communications 20 (2012) 68–70
69
Scheme 2. Catalytic acceptorless dehydrogenation of secondary alcohols.
(Scheme 2). We found that the ceramic material containing 3 mol% of
1 relative to the substrate is capable of inducing the desired transforma-
tions in refluxing mesitylene for 24 h in the presence of either homoge-
neous 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or heterogeneous
Cs2CO3 (3 mol%). Gasometric experiments have proven that an equimo-
lar amount of dihydrogen is evolved during the reaction. The reaction
mixture was found to contain solely the starting alcohol and
byproduct-free acetophenone. It is noteworthy that in the absence of
the base, the reaction is sluggish and does not reach complete conver-
sion even after a very long reaction period (3 days). Furthermore,
while the application of either DBU or Cs2CO3 in the reactions described
in Scheme 2 gave practically the same results, experiments with other
bases such as Bu3N or Et4N+OH− lead to considerably lower yields.
It should also be noted that although the non-heterogenized
iridium complex 1 is capable of catalyzing this reaction under lower
temperature (130 °C) and under a lower catalyst loading [26], it could
be employed just once. Entrapment within the sol–gel gives a recyclable
catalyst. For example, in a typical series of five consecutive dehydroge-
nation experiments of 1-phenylethanol with the same heterogenized
catalyst the yields of acetophenone were 88, 93, 89, 91, and 86%, respec-
tively. The slightly lower yield in the first cycle is rationalized by an
activation period of the pre-catalyst. After the fourth run visible mor-
phological changes are observed in the support of the catalyst, which,
apparently, allows a gradual minor leaching process.
Scheme 1. Proposed catalytic cycle for acceptorless dehydrogenation of alcohols by
iridium-based PC(sp3)P-metalated pincer catalyst.
content of the washings was b0.05 μg. Brunner, Emmet, and Teller
(BET) analysis of the heterogenized catalyst revealed a surface area of
723 m2/g. The average pore diameter was 44 Å according to Barett,
Joyner, and Halenda (BJH) measurements (see supporting information
provided herewith).
2.2. General procedure for the dehydrogenation of the alcohol
A 10 mL flask was charged with the sol–gel entrapped iridium
catalyst containing 3.32×10−2 mmol of complex 1, together with either
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 0.005 mL, 0.033 mmol) or
cesium carbonate (0.01 g, 0.033 mmol) and mesitylene (4 mL). The
mixture was stirred at 157 °C for 15 min at a velocity of 1000 rpm. The
alcohol to be dehydrogenated (1.1 mmol) was heated until GC and 1H
NMR indicated the desired conversion.
In several experiments the progress of the dehydrogenation
was recorded by attachment of the reaction system to a gas burette.
Removal of the mesitylene and isolation of the products were
achieved by column chromatography on silica gel using various
hexane–ethyl acetate mixtures as eluent.
In addition, we noticed that the yield of dihydrogen evolved in
the reaction catalyzed by the newly prepared matrix (first run)
slightly exceeds the theoretically calculated amount. As the blank
Table 1
Catalytic dehydrogenation of some alcohols by sol–gel entrapped 1a.
Entry Substrate
Time Products (yield, %)b
(h)
2.3. Recycling of the catalyst
1
2
3
4
1-Phenylethanol
1-(4-Bromophenyl)ethanol
1-(4-Methoxyphenyl)ethanol 41
1-(2,5-Dimethoxyphenyl)
ethanol
Benzhydrol
24
24
Acetophenone (93)c,d
4-Bromoacetophenone (89)d
4-Methoxyacetophenone (85)d
2,5-Dimethoxyacetophenone (90)d
When the dehydrogenation was completed, the entrapped cata-
lyst was filtered off, and washed with hot THF until the washings
were colorless. The washings were subjected to ICP analysis for deter-
mination of the Ir content. The solid recovered catalyst was dried at
80 °C for 24 h at 0.5 Torr and reused in the next catalytic run.
TEM-EDX analysis indicated that unlike many other catalytic
systems in which hydrogen transfer takes place, the iridium complex
is not converted into metallic nanoparticles during the process [31].
48
5
6
7
48
24
48
Benzophenone (88)d
2-Octanone (91)d
Benzyl benzoate (95)
Benzaldehyde (5)
2-Octanol
Benzyl alcohol
8
4-Methylbenzyl alcohol
4-Methoxybenzyl alcohol
2-Bromobenzyl alcohol
4-Chlorobenzyl alcohol
48
48
48
48
4-Methylbenzyl 4′-methylbenzoate
(76)
4-Methylbenzaldehyde (24)
4-Methoxybenzyl
4′-methoxybenzoate (65)
4-Methoxybenzaldehyde (19)
2-Bromobenzyl 2′-bromobenzoate
(82)
9
3. Results and discussion
10
11
The entrapment of the iridium compound 1 was carried out by
vigorous stirring of the complex in THF with pre-hydrolyzed TMOS
at room temperature until the mixture underwent complete gelation
(usually ca. 24 h). The resulting ceramic material was refluxed in
mesitylene and the filtrate was subjected to ICP-MS analysis in order
to estimate possible leaching of the entrapped complex. The measure-
ment detected b5 ppm of iridium in the filtrate after prolonged treat-
ment of the matrix with boiling mesitylene. This suggests that the
contribution of leached homogenous 1 to the catalytic dehydrogenation
may be neglected.
2-Bromobenzaldehyde (9)
4-Chlorobenzyl 4′-chlorobenzoate
(90)
12
13
1,4-Butanediol
Diethylene glycol
24
24
Butyrolactone (91)d
1,4-Dioxan-2-one (93)d
a
Reaction conditions: alcohol (1.1 mmol), iridium complex (0.0332 mmol) entrapped
within silica sol–gel prepared from TMOS (3.4 g), Cs2CO3 (10 mg, 0.033 mmol) and
mesitylene (4.0 mL); 157 °C.
b
The yields are the average of at least two experiments that did not differ by more
than 3%. The yields are based on GC and 1H NMR analyses.
c
Our catalytic experiments were focused on the dehydrogenation of
1-phenylethanol derivatives to form acetophenones and dihydrogen
In the second run.
Yields of isolated products.
d

70
K. Oded et al. / Catalysis Communications 20 (2012) 68–70
Acknowledgements
This research was supported by the Israel Science Foundation
Grants No. 13/11 (DG) and 229/10 (JB).
Scheme 3. Catalytic acceptorless dehydrogenation of primary alcohols.
Appendix A. Supplementary data
experiment performed in the absence of the alcohol also produces
some dihydrogen, we assume that this is associated with the dehy-
drogenation of residual MeOH generated during the preparation
of the sol–gel support. Upon recycling of the catalyst (second and
further runs) this “first run effect” disappears and the amount of H2
formed fits the calculated yield.
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2012.01.017.
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4. Conclusions
We demonstrated that sol–gel entrapped catalyst 1 is capable of
promoting acceptorless dehydrogenation of primary and secondary
alcohols. Although the dehydrogenations by the non-heterogenized
catalyst are faster than by the sol–gel encaged complex, recyclability
and significant stabilization of the organometallic species toward non-
inert conditions represent a clear advantage from the environmental
and practical points of view. Application of the heterogenized catalyst
for production of molecular hydrogen from more challenging substrates
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of the activity of the sol–gel supported catalyst will follow.
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